Vertical-cavity surface-emitting lasers (VCSELs) have several advantages such as low-threshold, small size, on-wafer testability and high fiber coupling-efficiency. Single-mode VCSELs would be especially useful for applications such as data—(emission wavelength of λ=850 nm) and telecommunication (λ=1.3-1.7 μm), if sufficient high single-mode output powers (5-20 mW) could be extracted from a VCSEL.
VCSELs emit radiation perpendicular to the semiconductor substrate plane, either from the top or the bottom of the device structure. A VCSEL is a surface-emitting laser having mirrors disposed parallel to the substrate surfaces that form and enclose an optical cavity between them. VCSELs usually have a substrate upon which a first mirror stack and second mirror stack are disposed, with an active region between them comprising either quantum wells or quantum dots. Since the gain per pass in a VCSEL is much lower than in an edge-emitting laser a higher reflectivity of the mirrors is required. For this reason, the mirror stacks in a VCSEL typically comprise a plurality of Distributed Bragg Reflector (DBR) mirrors, which may have a reflectivity of 99% or higher. An electrical contact is usually positioned on the second mirror stack, and another contact is provided at the opposite end in contact with the substrate. When an electrical current is injected to flow between the two contacts, lasing is induced from the active region and emits through either the top or bottom surface of the VCSEL. A schematic drawing of a VCSEL structure is shown in FIG. 1.
FIG. 1 illustrates an example of a VCSEL 100 according to the prior art. The core of the VCSEL 100 is an active region 110 that includes quantum wells 112. In FIG. 1, the active region 110 is bound by the upper DBR layers 108 and lower DBR layers 114. One of the DBR layers includes n-type semiconductor material and the other p-type semiconductor materials. DBR layers are formed by alternating layers of material whose refractive index varies. Each individual DBR layer typically has a thickness of approximately λ/4. These alternating layers are often formed from semiconductor materials or dielectric materials.
Light is reflected at the junction of the DBR layers, but in order to achieve the high reflectivity required by VCSELs, many layers must be formed or grown. Thus, the DBR layers 108 and 114 form mirror layers that reflect light through the active region 110. The light aperture of the VCSEL 100 through which light is emitted is typically formed by selective oxidation of one or some of the DBR layers (region 106) or ion implantation to form an aperture 107 through which light can escape and through which electrical current can flow. Finally, the VCSEL 100 also includes a substrate 116 and metal contacts 104 and 118.
The composition of the active region 110 is often related to the wavelengths that are generated by the VCSEL 100 and are typically formed from some combination of GaAs or AlGaAs. The present invention is not limited to these materials. For example GaNAs, InGaNAs, InGaAs, GaInNAs, InGaAsP, and InGaP are often used in quantum wells that emit wavelengths of 650, 780, 850, 980, 1300 and 1550 nm. The composition of the quantum well or the bulk active region has an impact on the band gap, which is related to the wavelengths or modes generated by the VCSEL 100.
The mode confinement in conventional optical waveguides is achieved by having a core with a high refractive index surrounded by a cladding with a lower refractive index. This results in a waveguide based on the principle of total internal reflection. In an optical resonator, such as a VCSEL, a cavity resonance-wavelength shift corresponds to an effective index step in a conventional optical waveguide structure, Δλ/λ=Δn/n, as pointed out by Hadley [G. R. Hadley, “Effective index model for vertical-cavity surface-emitting lasers,” Optics Letters Vol. 20, No. 13, p 1483 (1995)]. As a result, lateral mode confinement in a VCSEL can be accomplished by having a core with a long cavity resonance-wavelength surrounded by a cladding region with a short cavity resonance-wavelength. The lateral mode confinement in such a VCSEL is analogous to the lateral mode-confinement in a conventional index-guided fiber.
In contrast, micro-structured fibers having a rich topology in the refractive index of the cladding, typically air-holes in a silica cladding, enable lateral mode-confinement to a central, low-index core of the fiber (for example a large air-hole). These fibers are denoted Photonic Band-Gap (PBG) fibers. The PBG fibers guide light by an effect that may be seen as an optical analogue to the electronic bandgaps of semiconductors. The light aperture of a PBG fiber, also denoted the PBG-defect, has an effective index which is lower than the surrounding cladding region. A fiber where the index of the core is lower than the cladding is denoted an anti-guide, and would not confine any lateral modes in the absence of the photonic bandgap effect.
Lateral mode-confinement by the PBG effect in a VCSEL can be implemented by a core (light aperture) surrounded by a cladding region with a rich topology with variations of the cavity resonance-wavelength. Etching in the top of a VCSEL DBR top-mirror results in a shift of the cavity resonance-wavelength. Local etching of a VCSEL top mirror is thus an efficient method to locally modify the cavity resonance wavelength and to implement lateral mode confinement by the PBG effect as described in the previous patent application WO 02/073753.
Let us for clarity give a more in-depth explanation of the relation of the index step in an optical fiber with the cavity resonance condition in an optical resonator like a VCSEL.
An optical resonator can be realized using two plane-parallel mirrors separated with a spacer of a thickness L and refractive index n. Without loss of generality we can assume that the plane mirrors extend in the xy-plane. An optical resonator is characterized with a set of resonance wavelengths λm=2 Ln/m, m=1, 2, 3, . . . Light with these wavelengths will at normal incidence experience increased transmission through the resonator.
The laser cavity may support a plurality of lateral modes for the generated light. In the present application, a lateral electromagnetic mode can be regarded as a distribution of plane waves, with wave-vectors {right arrow over (k)} describing the propagation of the electromagnetic field. Any of the wave-vectors {right arrow over (k)} may be projected onto a plane which is normal to the extension of the cavity, such as a plane parallel to a mirror. The projection of {right arrow over (k)} onto such a plane will be designated the lateral component {right arrow over (k)}∥ of the wave-vector {right arrow over (k)}, the term lateral refer to the extent of the cavity rather than to {right arrow over (k)}. The length of {right arrow over (k)} is
      k    =                  2        ⁢        π        ⁢                                  ⁢                  n          _                    λ        ,where n is the effective longitudinal index of refraction. The lateral component of {right arrow over (k)} is thus useful for defining a lateral-wavelength:
            λ              =                  2        ⁢        π        ⁢                                  ⁢                  n          _                                              k                                        ,the propagation constant in the z-direction is accordingly
            k      z        =                  2        ⁢        π        ⁢                                  ⁢                  n          _                            λ        z              ,where k=√{square root over (|kz|2+|k∥|2)}.
Let us in a first example assume a VCSEL resonator where the core of the VCSEL has a short cavity resonance-wavelength λcore, while the cladding region has a longer cavity resonance-wavelength λclad. The lengths of the resonant wave-vectors of the two regions are:
      k          z      ,      core        =                              2          ⁢          π          ⁢                                          ⁢                      n            _                                    λ          core                    >              k                  z          ,          clad                      =                            2          ⁢          π          ⁢                                          ⁢                      n            _                                    λ          clad                    .      We first consider an electromagnetic field resonant with the core resonator: λ=λcore and
      k    =                  2        ⁢        π        ⁢                                  ⁢                  n          _                            λ        core              ,hence the field has a wave-vector {right arrow over (k)}={right arrow over (k)}z,core, and propagates solely in the direction of the z-axis in the core resonator. However, in the cladding resonator we find that since {right arrow over (k)} is conserved, {right arrow over (k)}={right arrow over (k)}z,clad+{right arrow over (k)}∥,clad, where k∥,clad2=k2−kz,clad2 or
      k                        ,        clad              2    =                    (                              2            ⁢            π            ⁢                                                  ⁢                          n              _                                            λ            core                          )            2        -                            (                                    2              ⁢              π              ⁢                                                          ⁢                              n                _                                                    λ              clad                                )                2            .      Since λcore<λclad the value of k∥,clad is real and the field can freely propagate in the cladding region. In this case the core region is an anti-guide. Now in the opposite case λcore>λclad the value of k∥,clad would be complex corresponding to an evanescent field in the cladding region. In this case the core would be a guide for the optical field.
The discussed example shows how the cavity resonance wavelength determines the guide- or anti-guide properties of a VCSEL. Furthermore, the discussed example gives a good physical picture of what is meant by lateral-wavelength (λ∥), which will be extensively used when discussing the implementation of lateral mode-confinement by the photonic band-gap effect in a VCSEL.
For PBG waveguides, the physical dimensions of the PBG micro/nano-structuring is related to the lateral wavelength (λ∥) and is expected to be quite large since λ∥ is expected to be much larger than longitudinal wavelength λz as well as the free-space wavelength. The realization of the PBG effect is thus not expected to be limited by the present limits of semiconductor processing techniques.
The cavity resonance wavelength of a vertical-cavity surface-emitting laser can be changed by a variation of the thickness of one or several layers within the Bragg mirror of the VCSEL. A change in the cavity resonance wavelength can be observed as a periodic function of etch depth, when etching through a VCSEL mirror.
As described previously, the lateral mode confinement of a waveguide (VCSEL) depends on the index (cavity resonance wavelength) difference. Local etching of a VCSEL top mirror can be used to form a weak index guide or a weak anti-guide, if the cavity resonance wavelength is longer or shorter for the core part of the VCSEL compared to the cladding region, respectively. The reflectivity of the VCSEL top or bottom mirror is changed when the mirror is thinned down by etching. However, the reflectivity is not decreasing monotonically as a function of etch depth, but is, similarly to the cavity resonance wavelength, a periodic function of the etch depth. The periodicity is given by the longitudinal quarter wavelength within the Bragg mirror material. The maximum and minimum reflectivity of this periodic mirror reflectivity is of course decreasing for increasing etch depth as a result of the decreasing number of DBR mirror layer pairs.
FIG. 2 illustrates an example of a vertical cavity surface emitting laser 200 where a photonic micro/nano-structure 210 is added in the top of the DBR mirror layers 202, see e.g. WO 02/073753. The active region 204 is bound by the upper DBR layers 202 and lower DBR mirror layers 206. The photonic micro/nano-structure 210 is formed by shallow or deep etched holes 212 from the top of the upper DBR mirror for lateral mode control. The photonic micro/nano-structure defines a light aperture region 214 through which light is emitted. The lateral current confinement is typically formed by selective oxidation of one or more layers within the DBR structure or ion implantation. The current aperture is typically larger than the light aperture 214 of the photonic micro/nano-structure. The lateral light and current aperture are thus decoupled for this kind of VCSEL 200. The lateral optical modes are thus optimised independently of the electrical lateral current aperture.
The cavity resonance wavelength shift is relatively small when etching in a complete semiconductor top-mirror in case of shallow etches (just a few DBR mirror pairs). The cavity resonance wavelength is changed by several nanometers, when deep holes (penetrating several periods) are etched into one of the DBR mirrors of the VCSEL as in region 210 of FIG. 2. In this case the longitudinal reflectivity of the etched areas 212 is significantly reduced as a result of the reduced number of DBR mirror layer pairs. The reduced DBR reflectivity will, together with the achieved cavity resonance wavelength shifts, determine the lateral modes confined to the light aperture 214. In some cases, the laterally changing loss will dominate the mode confinement in a process similar to the gain-guiding mechanism well known from broad area edge emitting lasers. Furthermore, the implementation of the PBG effect requires a relatively high cavity resonance wavelength shift between etched and un-etched areas.
The actual value of the cavity resonance wavelength as function of etch depth is dependent on the position of etches within the DBR. A large shift of the cavity resonance wavelength is possible by shallow etching, when the PBG structures are close to the active region of the VCSEL and embedded in the DBR top mirror. The maximum cavity resonance wavelength-shift decreases (for the same etch depth), as the position of the PBG structures are closer to the top of the DBR mirror. Hence, to introduce the necessary high cavity resonance wavelength shift between etched and un-etched areas, shallow holes 312 are etched within one of the VCSEL DBRs as shown in FIG. 3 instead of on top of the DBR. The cavity resonance wavelength shifts are very large (several tens of nano-meters) for etch depths of less than 200 nm, and the mirror reflectivity (photon-lifetime of the VCSEL cavity) is much less affected by this approach.
FIG. 3 illustrates an example of a vertical cavity surface emitting laser 300 where a photonic micro/nano-structure 318 is embedded in the top DBR mirror layers 302, see e.g. WO 02/073753. The shallow etching of holes 312 is done in a partial top mirror, which reflectivity is insufficient for lasing of the VCSEL. The top DBR mirror reflectivity necessary for reaching lasing is for example obtained by a re-growth processing step or deposition of a dielectric top-mirror. The full DBR top mirror 302 consists of three different layers, the first un-etched partial top-mirror region 316. The lateral micro/nano-structuring 310 in the partial top-mirror layer 318 is made by shallow etching of holes. The top-mirror 320 is deposited or re-grown after definition of the lateral micro/nano-structuring 310.
The partial DBR top-mirror layer 316 may be omitted or replaced with a spacer layer. Furthermore, the micro/nano-structured layer 318 may be formed by shallow etching in either a partial semiconductor DBR top-mirror layer or a dielectric DBR top-mirror layer or a combination of both semiconductor and dielectric partial DBR top-mirror layers. The light-aperture region 314 defines the PBG-defect to which region the light is confined. The cavity resonance wavelength of the LA-region 314 is typically at least shorter or equal to the surrounding region 312 and 313. The cavity resonance wavelength depends on the etch depth and the difference in cavity resonance wavelengths of region 312, 313 and 314 has been visualized by different etch depths. Finally, the VCSEL 300 also includes an active region 304, a lower DBR bottom-mirror 306 and substrate 308.
The patent U.S. Pat. No. 6,396,865 by L. J. Mawst and D. Zhou discloses a vertical-cavity surface-emitting laser, where the lateral mode confinement is implemented with a single or two anti-resonant reflecting optical waveguide (ARROW) rings. The ring width is an odd number of lateral quarter wavelengths.
The ARROW method using only a single ring was also published in: D. Zhou and L. J. Mawst, “Simplified-antiresonant reflecting optical waveguide-type vertical surface-emitting lasers”, Applied Physics Letters Vol. 76, No. 13, p. 1659 (2000). The ARROW method uses one or two anti-resonant reflecting rings to reduce the lateral wave-guiding losses resulting from the poor mode confinement of a central anti-guide. The anti-guide promotes single-mode operation, while the anti-resonant reflecting ring(s) reduce the lateral radiation losses for the fundamental mode and higher order modes. The threshold current of a VCSEL using only an anti-guide is thus reduced when implementing the ARROW mode confining ring(s). The lateral radiation losses for the higher order modes are also reduced in the ARROW design, but are still significantly higher than for the fundamental mode.
In “Suppression of Polarization Switching on Birefringent Antiresonant Reflecting Optical Waveguide Vertical-Cavity Surface-Emitting Lasers,” IEEE Photonics Technology Letters, Vol. 16, No. 3, p. 711 (2004) by N. S. Chen, S. F. Yu, and C. W. Tee, the one and two anti-resonant ring VCSEL designs are shown to have improved single polarization mode stability, when the ring next to the anti-guide (light aperture) is slightly broader than an odd number of lateral quarter wavelengths. The ring width increases up to approximately λ/3 for optimum polarization stability, the ring is circular and does not introduce any asymmetry in the lateral reflection coefficients for the two orthogonal polarization states.
These prior single-mode designs are based on a method, where the anti-guide (light aperture) is enclosed by an area (ARROW ring(s)) giving an increased confinement to the anti-guide, which reduces the lateral radiation losses of both the fundamental and higher order modes. The addition of these rings result in a decreased threshold current compared to an anti-guide VCSEL structure due to the reduced radiation losses. However, lateral mode confinement by one or two reflector-rings decreases the single mode operating regime for larger light aperture (anti-guide) size, since both the fundamental and higher order lateral modes are more strongly confined for these ARROW type VCSEL designs.
The physical implementations of these prior single-mode designs have been using epitaxial re-growth to implement the arrow rings. This is a costly production method and it reduces the yield significantly in the processing of devices. The designs furthermore use ion-implantation to confine current in the lateral direction. This has the advantage of restricting flow of current without introducing optical confinement. However, the method has to a large degree been abandoned by the industry due to large process variations and instabilities. Instead, oxide confinement is dominating present implementations of VCSELs resulting in high yield and excellent reliability. The introduction of oxide confinement in the arrow type single mode VCSELs is difficult and not yet demonstrated. The presence of the oxide layer will disturb the function of the arrow rings due to the lateral confinement of the oxide aperture itself.
U.S. Pat. No. 6,185,241 discloses a microcavity laser in which an annular metal layer suppresses higher-order modes by introducing an absorbing aperture. A typical value for the diameter of the annular metal layer is 5 um, and besides introducing a mode-dependent loss, the annular metal layer also defines the aperture of the laser.
U.S. Pat. No. 5,838,715 discloses a VCSEL comprising a loss-determining element that progressively increases an optical loss of the VCSEL's optical cavity with increasing lateral distance from the optical axis (the axis parallel to the substrate normal). An annular metal layer provides a mode-dependent loss and also defines the light aperture of the laser. The loss-determining element is typically a curved section that provides a lens effect.
U.S. Pat. No. 5,432,812 discloses use of a micro-cavity semiconductor laser having a three-dimensional optical reflector that covers a double-heterostructure section for controlling the spontaneous emission along various directions and for increasing the coupling ratio of spontaneous emission with a specific laser mode to thereby decrease the threshold current of the laser. Embodiments of lasers according to the invention disclosed in U.S. Pat. No. 5,432,812 may have a PBG-structure similar to those illustrated by 210 in FIG. 2 or region 310 in FIG. 3. A laser according to U.S. Pat. No. 5,432,812 has an optical reflector covering its double heterostructure section, which is a section that surrounds the active portion of the laser.